Anaerobic fermentations of hydrogen and carbon monoxide involve the contact of the substrate gas in an aqueous fermentation broth with microorganisms capable of generating alcohols such as ethanol, propanol, i-butanol and n-butanol. The production of these alcohols requires significant amounts of hydrogen and carbon monoxide. For instance, the theoretical equations for the conversion of carbon monoxide and hydrogen to ethanol are:6CO+3H2O→C2H5OH+4CO2 6H2+2CO2→C2H5OH+3H2O.
As can be seen, the conversion of carbon monoxide results in the generation of carbon dioxide. The conversion of hydrogen involves the consumption of hydrogen and carbon dioxide, and this conversion is sometimes referred to as the H2/CO2 conversion. For purposes herein, it is referred to as the hydrogen conversion.
Typically the substrate gas for carbon monoxide and hydrogen conversions is, or is derived from, a synthesis gas (syngas) from the gasification of carbonaceous materials, from partial oxidation or reforming of natural gas and/or biogas from anaerobic digestion or landfill gas or off-gas streams of various industrial methods such as off gas from coal coking and steel manufacture. The substrate gas contains carbon monoxide, hydrogen, and carbon dioxide and usually contains other components such as water vapor, nitrogen, methane, ammonia, hydrogen sulfide, and the like.
These substrate gases are typically more expensive than equivalent heat content amounts of fossil fuels. Hence, a desire exists to use these gases efficiently to make higher value products. The financial viability of any conversion process, especially to commodity chemicals such as ethanol, will depend, in part, upon the costs of the feedstocks, conversion efficiency and operating and capital costs for generating the substrate gases; and upon the capital costs, the efficiency of conversion of the carbon monoxide and hydrogen to the sought products and the energy costs to effect the conversion of the substrate gases to the higher value products.
In a bioreactor, hydrogen and carbon oxides pass from the gas phase to being dissolved in the aqueous broth, and then the dissolved hydrogen and carbon oxides contact the microorganisms for bioconversion. Due to the low solubilities of carbon monoxide and, especially, hydrogen in aqueous media, mass transfer can be a limiting factor rate and conversion in the bioconversion to alcohol. Therefore challenges exist in the design of commercial scale bioreactors that provide for the sought mass transfer while still enabling a high conversion of gas substrate at capital and operating costs that enable such a facility to be commercially competitive.
The off gases from bioreactors contain substrate that was not bioconverted and diluents such as methane, carbon dioxide, nitrogen, hydrogen sulfide, and other impurities. Also, the microorganisms present in the bioreactor metabolize components to impurities such as hydrogen sulfide, oxygen, nitrogen, hydrogen and other gases. The byproduct off gas can be combusted to produce heat and electricity, but this is not without concern. Specifically, hydrogen sulfide is toxic and if released to the atmosphere has a propensity to produce acid rain. Upon combustion of the raw off gas, the hydrogen sulfide present will form sulfur oxides, which are stack gas pollutants. Environmental regulations typically require removal and disposal of such stack gas pollutants. Commercial processes for removing sulfur before or after combustion are known; however, the treating of sulfur containing gas and sulfur recovery have significant cost and operating disadvantages that can impact commercial viability. Also, while various processes for sulfide recovery via alkali, for example sodium hydroxide, calcium hydroxide, and so forth, are known, the acidic nature of the off gas due to the co-produced carbon dioxide would result in excessive alkali consumption. More specifically, a lower and more acidic pKal of 6.37 for carbon dioxide compared to a pKal of 7.04 for hydrogen sulfide means increased alkali consumption to recover sulfide from the carbon dioxide containing bioreactor off gas. Accordingly, the costs associated with the equipment and its operation to recover sulfide by treating bioreactor off gas with alkali can make such recovery impractical.
Sulfur compounds, such as but not necessarily limited to, hydrogen sulfide, bisulfite, thiosulfate, and so forth, play a complex role in bioreactors and the effect on the microorganisms used in the anaerobic fermentation of hydrogen and carbon monoxide. The microorganisms that bring about such anaerobic fermentation generate very little metabolic energy, and do require some sulfur to maintain biological activity. Consequently, the relatively slow growth of the microorganisms, which often continue substrate fermentation during the non-growth phase of their life cycle to gain metabolic energy for their maintenance, can depend on available sulfur. While sustaining the microorganisms requires a certain presence of sulfur in the bioreactor feed, sulfur too much in excess of microorganism needs may be detrimental to microorganism activity for the anaerobic fermentation of hydrogen and carbon monoxide to liquid products.
Although known processing steps exist to remove hydrogen sulfide from bioreactor off gas to produce sulfide, large chemical consumption, particularly alkali, detracts from the commercial viability of the disclosed process.
Processes are therefore sought that can provide removal and conversion of off gas hydrogen sulfide to sulfur compounds that can be fed to the bioreactor to meet microorganism sulfur demand. Desirably such processes can operate at atmospheric pressure and low temperatures without the excessive cost of expensive chemicals and operate without the generation of hazardous and/or toxic wastes.